Meteorites from the outer solar
system?
Matthieu Gounelle1,2
1Laboratoire
d’Étude de la Matière Extraterrestre (LEME),
Muséum National d’Histoire Naturelle, Paris, France
2Impacts
& Astromaterials Research Center (IARC),
Department of Mineralogy, Natural History Museum, London, UK
Meteorites from the outer solar
system?
1. Introduction
2. The cometary orbit of the Orgueil meteorite
3. Direct comparison of carbonaceous chondrites and
comets
4. Implications & open questions
5. Conclusions
Meteorites from the outer solar
system?
1. Introduction
2. The cometary orbit of the Orgueil meteorite
3. Direct comparison of carbonaceous chondrites and
comets
4. Implications & open questions
5. Conclusions
Introduction
œ Meteorites are solid samples (rocks) from outer space
œ The origin of meteorites has been disputed since their identification as
extraterrestrial objects
œ First calculation from Biot (1803)
œ Only the orbit of 8 meteorites is known
œ All come from the asteroid belt
œ It is now widely accepted that
œ Most meteorites come from asteroids
œ 26 meteorites come from Mars
œ 32 meteorites come from the Moon
œ Are there any outer solar system (cometary) meteorites?
œ Nature of comets (pristine or processed?)
œ Relationship between comets and asteroids?
Meteorites from the outer solar
system?
1. Introduction
2. The cometary orbit of the Orgueil meteorite
3. Direct comparison of carbonaceous chondrites and
comets
4. Implications & open questions
5. Conclusions
Gounelle, Spurný & Bland (2006) Meteoritics & Planetary Sciences, 41, 135-150
The special nature of CI1 chondrites
œ Rare rocks (Orgueil, Alais, Ivuna, Tonk,
Revelstoke)
Orgueil (CI1)
œ CI1 chondrites represent 0.5 % of the
meteorites falls
œ Extremely porous rocks (up to 35 %)
œ Very dark rocks (low albedos)
œ CI1 chondrites strongly interact with the
atmosphere (formation of sulfate veins)
Gounelle & Zolensky 2001
The chemical composition of CI1 chondrites
œ Best chemical match with the
solar photoshere of all
chondrites
Secondary minerals in CI1 chondrites
Iron sulfide (Fe1-xS)
œAlthough chemically pristine, CI1
chondrites are the most hydrothermally
altered chondrites (serpentine,
saponite, sulfides, Ca-Mg carbonates,
magnetite)
30μm
Dolomite MgCa(CO3)2
Phyllosilicates
Serpentine+Saponite
The fall of the Orgueil meteorite
œ Orgueil meteorite fell May 18th 1864 in southern France
œ Fireball seen from north of France to north of Spain
œ 10 pages of visual observations published by Daubrée in the
main scientific journal of the time
œ Compte Rendus de l’Académie des Sciences de Paris
The Orgueil fireball observations
Letter of M. d’Esparbès à M. Le Verrier
At 8.13 pm, a prodigious luminous artifact inundated the city.
Everyone thought to be in the middle of the flames. It lasted a few
seconds, and was generated by something as large as the full Moon,
that crossed the sky as a shooting star would, abandoning a bluish
fiery trail. This trail disappeared slowly, and the sky became serene
again; however ten minutes after, one could still observe an immobile
cloud.
Roughly two minutes after this electric light was produced, a
detonation similar to that of a canon, and lasting from 80 to 100
seconds was heard.
It was a delightful May evening. The weather was superb.
Published by Daubrée, Compte-rendus de l’Académie des Sciences
de Paris, vol 58 (1864)
Observations used
for trajectory and
orbit determination
More northern
observation
Meteorites fall
Daubrée 1867
Orbit calculations made by
P. Spurný
The Orgueil fireball observations
The Orgueil fireball observations
œ From all these observations reported by Daubrée in the Compte-rendus,
we (Pavel Spurný)
œ Calculated the atmospheric trajectory of the Orgueil meteor
œ Calculated the orbit of the Orgueil meteorid
The orbit of the Orgueil meteorid
œ There are 6 orbital parameters to determine
œ The inclination, argument of perihelion, longitude of ascending node,
perihelion distance, time of perihelion passage are precisely determined
œ The aphelion distance (Q) depends on the fireball entry velocity
vboundary = 17.8 km.s-1
œ The entry velocity can be
related to the fireball
duration, t
œ If t<10 s, the orbit is
cometary
The fireball duration
œ M.Lajous (CRAS vol 58 page 1067)
“The time during which the fireball moved between the two observed points has
been evaluated to be 3 seconds”
œ M. de SaintAmans (CRAS vol 58 page 1069)
"The duration of the fireball apparition has been a few seconds at most“
œ M. Laurentie (CRAS vol 58 page 1069)
"Its velocity was less than a shooting star: the duration of its apparition was no
more than 5 to 6 seconds"
œ Quote from a Perigueux newspaper (CRAS vol 58 page 1070)
"The duration of the phenomenon has been of a few seconds"
œ M. Triger (CRAS vol 58 page 1071)
"Duration of the phenomenon was a few seconds"
œ M. Hende (CRAS vol 58 page 1071)
"Duration of its fall was evaluated to be 5 to 6 seconds"
The orbit of the Orgueil meteorid
œ The most common observation is of a few seconds
œ The longest estimate is 6 seconds
œ Question: would observers distinguish a few seconds from 10 s?
œ Answer: probably
œ The good agreement between six different independent observers strengthen the
validity of observations (coherent observers)
œ Observers are casual but concerned enough by science development to report to the
Académie their observations (trustworthy observers)
œ The language used is very assertive (“No more than 5 to 6 seconds”, reflexive
observers)
œ Intense signals tend to be surestimated (Fraisse, Annual Review of Psychology 1984)
œ All observations point towards t <10 s and v > 17.8 km.s-1
œAll observations favour a cometary origin for Orgueil [most probable solution but
not certainty]
œRelated meteorites could also come from comets
œCI1 chondrites
œTagish Lake (ungrouped C chondrite)
Meteorites from the outer solar
system?
1. Introduction
2. The cometary orbit of the Orgueil meteorite
3. Direct comparison of carbonaceous chondrites
and comets
4. Implications & open questions
5. Conclusions
Comparison between comets and CI1
chondrites 1. Chemistry
œ Comets are assumed to be chemically
unfractionated samples from the
accretion disk. So is Orgueil
œ Orgueil (and Ivuna) is rich in amino
acids such as β-alanine and glycine
which can be made by HCNpolymerization (Ehrenfreund et al.
2000). Comets are rich in HCN
œ Comets contain PAHs, pure C grains
and complex organic matter as
carbonaceous chondrites do
Ehrenfreund et al. 2000
Comparison between comets and CI1
chondrites 2. The D/H ratio
œ The D/H ratio of comets is usually considered
to be higher than the D/H ratio of chondritic
material
œ This would contradict the cometary origin of
CI1 chondrites
BUT
Eiler & Kirchen 2004
œ The D/H ratio of CI1 chondrites probably
largely surestimated because of terrestrial
contamination
œ When 2σ error bars are plotted, only comet
Halley is significantly different from CI1
chondrites
Also do not forget that comparison is made between the gas phase (comets) and
solid phase (chondrites) isotopic composition. Possible fractionation?
Comparison between comets and CI1
chondrites 2. The D/H ratio
2σ error bars
Hale Bopp
Hyakutake
Halley
CI chondrites
Machholz
SMOW
Comets data from Meier et al. (98), Eberhardt et al. (95), Bockelée-Morvan et al. (98), Crovisier et al. (05)
CI1 data from Kerridge (1985) and Eiler & Kitchen (2004)
Comparison between comets and CI1
chondrites 3. Spectroscopy
œ Meteorites and cometary nuclei spectra are difficult to compare
œ
œ
œ
œ
Few bare nuclei spectra
Space weathering of comets
The CI1 chondrites have interacted with the atmosphere (extra features?)
Tagish Lake’s spectrum compares well to comet C/2001 OG108 and very well
to D asteroids
Comet C/2001 OG108
Tagish Lake
Abell et al. 2005
Kanno et al. 2003
Meteorites from the outer solar
system?
1. Introduction
2. The cometary orbit of the Orgueil meteorite
3. Direct comparison of carbonaceous chondrites and
comets
4. Implications & open questions
5. Conclusions
Comets are no longer primitive samples
œ For a long time, comets were envisoned as an aggregate of primitive interstellar matter
œ CI1 chondrites have been hydrothermally altered: so comets
œ Interaction between water and anhydrous minerals such as olivine, pyroxene produce secondary
minerals (phyllosilicates, carbonates, sulfides…)
œ Hydrothermal activity in comets is not surprising…
œ Comets are rich in water…
œ Possible heat sources are impacts or extinct radioactive decay (26Al…)
œ The discovery of craters at the surface of comet Wild 2 and of CAI fragments among Stardust
samples strengthen both possibilities
Picture released by M. Zolensky, Stardust
Mineralogy subteam leader, LPSC 2006
Wild 2, Brownlee et al. 2004
There is an asteroid-comet continuum
Asteroids
Comet Borrelly (Britt et al. 2004)
œ Existence of ambiguous/variable objects
œ 3200 Phaeton and 4015 Wilson-Harrington
œ Main Belt Comets (Hsieh & Jewitt 2006)
œ Difference between a water-rich
asteroid and a dust-rich comet?
œ Dust/gas of comets > 1, i.e comets are
rocky objects
œ Asteroid Ceres is rich enough in water to
have differenciated (Thomas et al. 2005)
Comets have geological features: rocks?
Open question:
Where do anhydrous IDPs come from?
œ Anhydrous IDPs are a subset of the Interplanetary Dust
Particles collected in the stratosphere by NASA
œ They are mainly made of olivine and pyroxene
œ They do NOT contain hydrated silicates or carbonates
œ For a long time, it has been argued they come from comets
œ Because they have large hotspots of D enrichments
œ Because they are friable (see picture)
œ D enrichments are now found in carbonaceous chondrites
(Busemann et al. 2006)
œ The friable nature of cometary dust is uncertain
15 microns
œ There is no definitive argument for the cometary origin
of IDPs
œ Anhydrous IDPs AND hydrated carbonaceous chondrites
could both come from comets as there are probably many
different types of comets
Busemann et al. 2006
Open question:
What do Stardust samples tell us?
œ The NASA mission Stardust brought back to Earth in January 2006
samples from the Jupiter Family Comet Wild 2
œ Some nanograms of samples have been analyzed so far
œ No presence of phyllosilicates and carbonates so far
œ Unlike CI1 chondrites, unlike Tagish Lake
œ Does it mean that CI1 chondrites cannot come from comets?
œ The answer is NO. Stardust sampled the surface of one comet
œ Comets are probably different one from the other
œ Altered rocks probably made in the interior of the comet (e.g. Young et al.
1998)
œ Phyllosilicates and carbonates detected in the ejecta of Tempel 1 (Deep
Impact mission, Lisse et al. 2006)
Meteorites from the outer solar
system?
1. Introduction
2. The cometary orbit of the Orgueil meteorite
3. Direct comparison of carbonaceous chondrites and
comets
4. Implications & open questions
5. Conclusions
Conclusions
œ The orbit of the Orgueil CI1 chondrite is compatible with that of a
comet rather than with that of an asteroid. Could Orgueil be the first
identified cometary meteorite?
œ When directly compared, comets and CI1 chondrites (as well as Tagish
Lake) exhibit some similarities (and differences)
œ Only comet Halley has a significantly higher D/H ratio than CI1
chondrites
œ CI1 chondrites endured severe hydrothermal alteration. So did comets?
Comets are NOT primitive aggregates of interstellar matter
œ There is probably a continuum between dark asteroids and comets
typified by CI1 chondrites, Tagish Lake and CM2 chondrites
œ Some open questions remain
œ Origin of IDPs
œ Stardust samples vs CI1 chondrites